1. Field of the Invention
The present invention relates to a developing apparatus and an image forming apparatus, and more particularly, it relates to a developing apparatus and an image forming apparatus each including a toner carrier for supplying toner to an electrostatic latent image formed on an image carrier.
2. Description of the Background Art
An image forming apparatus including a developing apparatus including a toner carrier for supplying toner to an electrostatic latent image formed on an image carrier is known in general, as disclosed in Japanese Patent Laying-Open No. 2008-107442, for example.
The aforementioned Japanese Patent Laying-Open No. 2008-107442 discloses an image forming apparatus including a developing apparatus including a photosensitive belt forming an electrostatic latent image, a developing unit including a developing roller for supplying toner to the electrostatic latent image formed on the photosensitive belt, a toner cartridge storing the toner and having a passage supplying the toner to the developing unit, a control valve arranged in the developing unit for opening/closing the passage of the toner cartridge and a transport paddle controlling opening/closing of the control valve. In the developing apparatus of the image forming apparatus, the transport paddle is configured to open the control valve not only to supply the toner from the toner cartridge toward the developing unit but also to supply (eject) the toner from the developing unit toward the toner cartridge.
In the developing apparatus of the image forming apparatus described in the aforementioned Japanese Patent Laying-Open No. 2008-107442, however, the transport paddle opens the control valve also to supply (eject) the toner from the developing unit toward the toner cartridge, and hence old toner remaining in the developing unit may be returned toward the toner cartridge, to come into contact with new toner. In general, toner is positively or negatively charged in a normal state, while the quantity of charge of the toner is reduced following the lapse of time or due to adhesion to the developing roller, leading to deterioration of the toner. When such deteriorated old toner comes into contact with new toner, the new toner so removes charge from the old toner that the old toner is reversely charged (negatively charged when having been positively charged in the normal state, and vice versa). When the reversely charged old toner is supplied to the photosensitive belt through the developing roller, the old toner adheres to a region of the photosensitive belt where no electrostatic latent image is formed. Consequently, the old toner adheres to a region of a printing paper irrelevant to the electrostatic latent image, to disadvantageously cause smudging (scumming) on the printing paper.